The present invention relates to the casting of metal alloys and other materials in the making of dental restorations such as crowns and bridges, and in particular to methods and apparatus for pressure casting.
In a commonly practiced method for making dental restorations, a wax pattern is obtained by applying the wax directly to the prepared tooth and then manually shaped by the dentist so that it has the proper contour and size. The shaped wax pattern is then removed from the tooth by means of a sprue pin, washed and dried. Since the final product which is to be permanently mounted on the tooth will have the same shape as the wax pattern, a mold, called an investment, must be made. A metal ring open at both ends is selected which must be large enough for the wax pattern to fit inside without touching the sides or extending to either end of the ring. An asbestos liner is often used to allow for thermal and hydroscopic expansion of the investment material and to serve as an insulator against the loss of heat during the casting process.
After the wax impression is placed within the casting ring, investment material, such as a mixture of plaster of Paris and some form of silica, is carefully packed around the pattern. A characteristic of investment materials is that they undergo expansion to compensate for the shrinkage of the casting as it cools. For example, the shrinkage of gold alloy is approximately 1.25% and if this were not compensated for, the resultant casting would be too small for a proper fit. The expansion of the investment material occurs in two phases, the first taking place as the material hardens and the second occurring when the investment is subjected to high temperature heat.
When the investment has set, the wax pattern is removed by placing the investment in a high temperature oven which causes the wax to melt and then volatize. As the investment is heated, it will undergo the aforementioned thermal expansion so that the mold cavity will be slightly larger than the wax pattern. After the invested ring is removed from the furnace, the casting is normally made within one minute or less before the investment cools and undergoes shrinkage. Although specific examples of prior art techniques for casting dental restorations will be discussed hereinafter, in general, casting comprises the forcing of a molten alloy such as a gold alloy into the chamber formed by the wax impression, permitting the casting to cool and then removing it from the investment.
The surface tension of the molten casting material is an important factor in making dental castings since it cannot effectively be overcome by gravity alone and the transfer of the metal to the mold is therefore not possible unless some additional force is exerted. In one method of pressure casting, compressed gas is applied to the molten metal by sealing the space above the metal which is supported on the ring encased investment and then admitting a compressed gas such as air, CO.sub.2 or N.sub.2 to this space.
In a second prior art technique, the gold alloy is placed on the investment above the cavity and a vacuum is applied to the bottom of the investment, which is porous in nature. The gold is heated by means of a torch and as it begins to melt, is drawn down into the mold cavity by the vacuum. Aside from the adverse effects which flame melting may have on the alloy, it is difficult to obtain a good vacuum on the investment so that often the material is not drawn down sufficiently to fill all the cavities of the mold.
A technique recently in use comprises placing the gold in a carbon crucible which is positioned beneath an inverted investment mold. The material is melted within the crucible and then the mold and crucible are simultaneously inverted 180.degree. so that the molten material runs slowly into the mold. Compressed air is then admitted quickly to the casting chamber surrounding the crucible and mold to assist in filling the mold. The problem with this technique, however, is that the metal cools before it fills all the interstices of the mold cavity.
Another technique which is in common practice today is that of centrifugal casting wherein the mold is spun to develop the centrifugal forces necessary to force the molten metal into the mold. A severe disadvantage of centrifugal casting, however, is that the forces in the casting ring vary in different directions so that different areas of the mold cavity will be filled with greater difficulty than others. There is also some danger to this technique because the molten gold will occasionally be thrown from the mold as it spins thereby burning the technician. Such an incident also results in the loss of the gold which in itself is a very serious matter in view of the rapidly increasing cost of this metal.
Modern gold alloys used in making dental restorations often have a number of components such as platinum, palladium, silver, copper, tantalum, indium, and iridium to impart hardness, high chemical resistance and fine grain structure, but as gold is melted, it tends to purify itself by expelling the other metals alloyed with it. Because of this characteristic, it is important that the metal not be heated for a prolonged period of time but that the casting be made as soon as possible after the optimum degree of melting has been obtained. Another situation which must be avoided is that of oxidizing the gold either by using an oxidizing flame or by heating too long in an atmosphere in which oxidation can occur. This may be avoided to a certain extent when using a blow torch for heating by using only the reducing portion of the flame; and if the gold is contained within a crucible, a reducing atmosphere may be used.
Other things which are deleterious to the gold include breakdown of the carbon crucible which gives off a residue which may mix with the gold causing pits, and the presence of contaminant gases such as sulfur dioxide and sulfate gases and residues from the wax pattern which tend to collect within the mold cavity. Although the vacuum casting technique tends to remove some of these contaminant gases, the negative pressure is not able to satisfactorily purge the cavity.
One aspect of making dental castings, which was alluded to earlier, is the importance of balancing the expansion of the investment material as it is heated with the shrinking of the gold as it cools. If the casting is too large for the prepared tooth, a greater margin of cement must be relied on to achieve an acceptable fit. If the cement margin is imperfect, however, it will dissolve and the tooth will be lost from decay. If the inlay is too small, subsequent grinding and polishing or partial dissolution in an acid often results in an imperfect fit which again may cause discomfort to the patient and eventual tooth decay.
To assure that the investment expansion and casting shrinkage are properly balanced, it is necessary that the casting be made while the investment is at a very high temperature and preferably that little or no cooling is permitted to occur. The length of time which occurs between removing the investment from the furnace and admitting the molten casting material into the mold, as well as premature cooling of the casting material are two of the disadvantages of the prior art casting techniques and apparatus.